The function of the casting powder in the continuous casting of steel is very complex. The casting powders first act as mould-release agents or as lubricants between the mould and the billet of steel. They also serve to cover the surface of the steel bath on the mould to prevent the surface oxidation of the steel bath due to the entry of atmospheric oxygen and further to reduce the heat loss from the surface of the steel bath. Thus, the flow bahavior of the casting powder is determinant with regard to the properties of the lubricant required, as is the melting behavior of the powder when it has been applied to the surface of the steel bath. The powder must be prevented from melting too rapidly because the unmelted, loose porous structure provides particularly good heat insulation properties.
Thus, on the surface of the steel melt, the casting powder passes through all the physical states from a loose, pourable powder to a flowable melt. Undesirable sintering effects, which occur in the transition region, produce large continuously porous sintered agglomerates which are very slow to make the transition into the flowing range because of the good heat insulation properties thereof. Sintered agglomerates of this type can also enter between metal shell and mould in an unmelted state and cause the steel billet to have surface defects, inclusions and a rough surface. In extreme cases such unmelted agglomerates can even bread open the outer shell of the billet and thus stop production. Thus, those skilled in the art are aware of the detriment caused by large agglomerates of casting powder and have sought to achieve compositions which have a low agglomeration tendency.
One known method of reducing the formation of agglomerates involves using multiphase systems as casting powders which contain a mixture, an inorganic oxidic material and particles substantially of carbon, which prevent the inorganic oxidic materials from melting together too rapidly.
The inorganic oxidic material is generally based on calcium silicate and oxidic raw material which may melt together on the surface of the steel bath or be used in the form of substantially homogeneous particles which have been pre-melted, fritted and milled. Aluminium oxide, titanium dioxide, iron oxide, manganese oxide, alkali and alkaline earth metal oxides and fluorine compounds, in addition to calcium oxide and silicon dioxide may be used as components of the inorganic oxidic material.
According to the prior art, graphite powder, mineral coal powder or lignite or carbon black are admixed with the inorganic oxidic material as the material substantially of carbon. The purpose of the carbon-containing material is to delay the sintering and melting together of the inorganic oxidic powders on the surface of the steel bath so that the desired porous layer which prevents the heat loss can be maintained. Atmospheric oxygen is simultaneously prevented from being admitted to the surface of the steel bath by combustion with the carbon-containing material to produce carbon oxide.
The quantitative ratio of inorganic oxidic and carbon-containing material is substantially determined by the cross section of the mould. Up to 25% by weight of carbon is contained in the casting powder in very small billet or bloom cross-sections. From 3 to 8% by weight of carbon in the casting powders suffices in very large slab cross-sections. The relatively high contents of carbon which were hitherto used in the casting powder frequently caused undesirable peripheral carburizing of the billet steel shell and flake cracks in the region of oscillation marks. Proposals have already been made to substitute the carbon particles in the casting powder with nitride particles (such as boron nitride) with the simultaneous addition of metallic particles (such as aluminium) as a reducing agent for the atmospheric oxygen ( U.S. Pat. No. 4,038,067).
However, casting powders of this type have not yet found their way onto the market.